This invention relates to profile extrusion of thermoplastic polymers to form shaped articles commonly referred to as profiles. More particularly, this invention relates to processes of profile extrusion utilizing certain polyester compositions.
Thermoplastic polymers are commonly used to manufacture various shaped articles which may be utilized in applications such as automotive parts, food containers, signs, packaging materials and the like. Profile extrusion is a common, cost-effective method for producing these shaped articles. Polymers such as polyvinyl chloride (PVC), acrylics and polycarbonates are typically used in profile extrusion. Each of these polymers suffers from one or more disadvantages. PVC is undesirable from an environmental standpoint since PVC produces toxic gases during melt extrusion and is difficult to dispose of after use. Acrylic articles are brittle and shatter when dropped or struck against another object. Polycarbonate is difficult to work with from a processor""s perspective and is too expensive for many applications. Polyesters, being notoriously difficult to process compared to many other polymers, have not been utilized as often in profile extrusion. As compared to polymers typically used in profile extrusion, polyesters have lower melt strengths and insufficient shear thinning resulting in a greater propensity for melt fracture if extruded at high output rates or low temperatures. Both melt strength and shear thinning are extremely important from the standpoint of profile extrusion.
Profiles are defined herein by a combination of two factors: shape and process of manufacture. The shape of a profile has a particular two-dimensional cross-section and an infinite length. The process of manufacture is known as profile extrusion. The cross-section lies in the x-y plane and the length lies along the z-axis. The x-y plane usually corresponds to the face of the die, whereas the z-axis corresponds to the extrusion or xe2x80x9ctake-offxe2x80x9d direction. Profiles can take on a wide variety of cross-sections varying in size, shape and complexity. Common xe2x80x9csimplexe2x80x9d profile shapes include hollow tubes, solid round stock, square cross-section stock, etc. More complex shapes such as those used for pricing channels, corner guards, and house siding can also be made.
By this use of shape as part of the profile definition, fiber, film and sheet might also be considered as special classes of profiles. Fibers have very small circular cross-sections and are extruded continuously in one direction. Film and sheet have rectangular cross-sections and are extruded continuously. However, in the industry as a whole and as defined herein by the additional definition factor of process of manufacture, film, sheet and fiber are not profiles because of how they are manufactured. Film or sheet, while infinite in length, are manufactured by processes that include the use of calendering or chill rolls. Fiber processes involve very high drawdowns, along with spinning cabinets and godet rolls. Profiles, in the industrial vernacular, represent constant cross-section, axially extruded structures, which have axial rigidity and are not wound. Profiles are usually cut to length and bundled, stacked or otherwise bound for transport. This axial rigidity obviously has important implications for what kind of xe2x80x9chaul-offxe2x80x9d equipment is used to convey the extruded product. Furthermore, the issues of melt strength and melt fracture are not important factors in fiber, film and sheet due to the nature of the take-up/winding equipment and the fact that shape definition is already trivial. Thus, as defined in the industry and herein, xe2x80x9cprofilexe2x80x9d shall not include fiber, film and sheet.
Profiles are fabricated by melt extrusion processes that begin by extruding a thermoplastic melt through an orifice of a die forming an extrudate capable of maintaining a desired shape. The extrudate is typically drawn into its final dimensions (along the z-axis) while maintaining the desired shape (in the x-y plane) and then quenched in air or a water bath to set the shape, thereby producing a profile. In the formation of simple profiles, the extrudate preferably maintains shape without any structural assistance. With extremely complex shapes, support means are often used to assist in shape retention. In either case, the type of thermoplastic resins utilized and its melt strength during formation is critical. Melt strength is defined as the ability of a polymer to support its weight in the molten state. For example, when extruded vertically from a die, a polymer with low melt strength will quickly sag and hit the floor; whereas, a polymer with high melt strength will maintain its shape for a much longer amount of time.
There are a number of quantitative and qualitative means for measuring melt strength. One standard test is disclosed in U.S. Pat. No. 4,398,022 wherein melt strengths for a polyester used in extrusion blow molding processes were measured at values between xe2x88x9210 and 10 percent. This same test is utilized herein and involves vertically extruding the polymer from a 0.1 inch (0.25 cm) diameter capillary die that is 0.25 inches (0.64 cm) long at a shear rate of 20 sxe2x88x921 up to a total length of 19 inches (49 cm). At this point the strand is cut near the die face and allowed to cool at room temperature. The diameter 6 inches (15 cm) from the end of the extrudate is then measured and expressed as a percentage change relative to the capillary diameter to give the melt strength. For example, if the strand diameter at a point 6 inches (15 cm) from the bottom was 0.12 inches (30 cm), then the polymer melt strength at that given melt temperature would be 20 percent (i.e. MS=(0.12xe2x88x920.1)/0.1*100 percent). Similarly, the xe2x80x9cdie swellxe2x80x9d is obtained by measuring the diameter xc2xd inches (1.3 cm) from the bottom of the extrudate and expressing it as a percentage change relative to the capillary diameter.
Polyesters due to their poor melt strength may have a negative value for the melt strength since the 6 inches (15 cm) point diameter could be less than the nominal diameter. For example, linear poly(ethylene terephthalate) modified with 1,4-cyclohexanedimethanol (PETG) having an inherent viscosity (IV) of 0.76 dl/g has been observed to have a melt strength of xe2x88x924 percent at 200xc2x0 C. and xe2x88x9224 percent at 220xc2x0 C. This means that the diameter of the extrudate measured 6 inches (15 cm) from the end of the strand was 4 percent smaller (200xc2x0 C. sample) than the die opening. Typical melt strengths for PVC under standard processing conditions (160 to 200xc2x0 C. processing temperature) are in the order of 20 to 30 percent. To achieve this melt strength with linear PETG would require an IV of around 0.95 dl/g. Thus, for applications in which melt strength is critical, polyesters will often not supplant these competitive polymers.
Another common melt strength test involves measuring the time period that an extrudate takes to reach a predetermined length below a die for a given flowrate/shear rate. While not standardized, this test provides an easy method for material comparison on a typical processing line and is used in some of the examples cited herein. Other non-standard melt strength tests such as measuring the degree of drooping in a horizontal profile extrusion line can also be applied giving a more application specific measure of melt strength.
Profile extrusions are usually run horizontally, and thus melt strength is important to minimize the amount of xe2x80x9cdrawdownxe2x80x9d and gravity-induced sagging the polymer experiences upon exiting the die. Drawdown is defined in profile extrusion as the amount of thickness reduction between the die and the take-up system and is expressed as the nominal thickness or width dimension at the die divided by the same dimension in the final part). For example, a typical polyester drawdown is about two. This means that the width of the final part is xc2xd that of the width at the die exit. Similarly, the final thickness is xc2xd of the thickness at the die exit. The take-up force of the puller or winder causes drawdown as the melt exits the die. A higher melt strength reduces the amount of drawdown, since there is greater resistance to stretching and thinning. For example, the drawdown for higher melt strength PVC is more on the order of 1.25. Reduced drawdowns make designing the appropriate dies and maintaining critical final part dimensions much easier.
The inadequate melt strength of polyesters further results in severe processing problems when polyesters are processed at typical profile extrusion temperatures of 390-550xc2x0 F. (200-290xc2x0 C.) and line speeds. Processing line speeds vary considerably depending on the shape of the profile. Typical speeds for simple shapes like a corner guard may be from 50 to 70 feet (15 to 20 meters) per minute. More complicated shapes may have process line speeds as low as one foot (0.3 meters) per minute, whereas extremely simple shapes with certain types of processing technology may run at speeds as high as 100 feet (30 meters) per minute. At the higher speeds, which obviously would be preferred by profile manufacturers, inadequate melt strength produces an extrudate that does not maintain its shape prior to quenching, and thus deformation occurs. To increase the melt strength of the polyester, processing temperatures are often lowered. This, however, increases the likelihood of an undesirable phenomenon known as melt fracture, which can only be eliminated by lowering the extrusion speed. By decreasing speed, the economic attractiveness of using polyesters is also decreased. Thus, the profile extrusion processes are often operated at maximum speeds associated with the highest temperatures and minimal melt strengths for maintaining particular profile shapes. Any increase in speed or lowering of temperature may cause an increase in high shear viscosity in the die, which then may cause an undesirable melt fracture.
From a Theological standpoint, melt strength depends primarily on the viscosity and, to some degree, on the elasticity or relaxation time of the melt. A higher viscosity increases the resistance to drawdown/sagging. In contrast, melt elasticity causes an increase in die swell, which serves to offset some of the effects of drawdown. Even though the same amount of width/thickness reduction is occurring after the die, the highly elastic material starts with a much higher initial width/thickness due to the greater die swell. Thus, the final part dimensions remain closer to the die dimension and the effective drawdown seems lower (thereby making it easier to design the tooling needed).
A viscosity curve for a given polymer has two regions of interest, as shown in FIG. 1. At very low shear rates the viscosity is highest and this is referred to as the xe2x80x9czero shear viscosityxe2x80x9d, xcex70. The zero shear viscosity (along with the elasticity) defines the melt strength since the polymer is experiencing essentially a zero shear rate after exiting from the die. Thus, the higher the zero shear viscosity, the higher the melt strength. In the high shear rate region, the polymer is xe2x80x9cprocessedxe2x80x9d with shear rates in the die/extruder ranging anywhere from about 10 sxe2x88x921 to 1000 sxe2x88x921. As low of a viscosity as possible in this range is desired in order to minimize pumping pressure and melt fracture. Fortunately, most polymers exhibit at least some degree of viscosity reduction or xe2x80x9cshear thinningxe2x80x9d at higher shear rates, which aids in their xe2x80x9cprocessabilityxe2x80x9d. Without the shear thinning, an extruder running a high melt viscosity polymer would require extremely high motor loads and/or very high melt temperatures, both of which can lead to polymer degradation and excessive energy consumption. In general, condensation polymers like polycarbonates and polyesters have a very low degree of shear thinning relative to addition type polymers like PVC and polyolefins. This is because the condensation polymers typically have narrower molecular weight distributions in addition to lacking the high molecular weight xe2x80x9ctailxe2x80x9d common in many addition polymers. This narrow molecular weight distribution makes polyesters more xe2x80x9cNewtonian-likexe2x80x9d (i.e. having a flat viscosity which does not depend much on shear rate) and characteristically harder to process.
Having a low viscosity at high shear rates (i.e. in the die) also serves to minimize the formation of melt fracture or xe2x80x9csharkskinxe2x80x9d on the surface of the extruded part or article. Melt fracture is a flow instability phenomenon occurring during extrusion of thermoplastic polymers at the fabrication surface/polymer melt boundary. The occurrence of melt fracture produces severe surface irregularities in the extrudate as it emerges from the orifice. The naked eye detects this surface roughness in the melt-fractured sample as a frosty appearance or matte finish as opposed to an extrudate without melt fracture that appears clear.
Melt fracture occurs whenever the wall shear stress in the die exceeds a certain value (typically 0.1 to 0.2 MPa). In turn, shear stress is controlled by the volume throughput or line speed (which dictates the shear rate) and the viscosity of the polymer melt. By reducing either the line speed or the viscosity at high shear rates, the wall shear stress is reduced and the chance for melt fracture is lowered. Thus, by increasing the degree of shear thinning, the viscosity is reduced at high shear rates, which then allows higher line speeds before melt fracture occurs.
Coupling all of these desired properties together, the ideal polymer for profile extrusion clearly will have a high zero shear viscosity in conjunction with a high degree of shear thinning. This maximizes melt strength while at the same time minimizes melt fracture and die pressures. A representative xe2x80x9cidealxe2x80x9d curve is illustrated in FIG. 1.
With respect to polyesters, either melt strength may be increased or melt fracture reduced without significantly affecting a change in the other. For example, by increasing the molecular weight or inherent viscosity of the polyester or by lowering the melt temperature, the zero shear viscosity will increase significantly along with the melt strength, but the degree of shear thinning will only change slightly. Thus, the melt strength will increase, but melt fracture will become even more of a problem since the high shear rate viscosity also increased significantly. This is in fact a problem for profile extrusions with polyesters and copolyesters. For example, in order to achieve an acceptable level of melt strength (i.e. greater than approximately 0 percent melt strength), PETG must be extruded at a melt temperature of 200xc2x0 C. or lower. This is an extremely low processing temperature for a polymer normally designed to be run at 220xc2x0 C. or higher. When run at this low temperature, PETG exhibits severe melt fracture even at low line speeds because the high shear rate viscosity is so high. This is due to the lack of significant shear thinning in PETG. Thus, with polyesters the process must be operated at either (1) hotter melt temperatures resulting in no melt fracture but too low melt strength or (2) low temperatures resulting in adequate melt strength but problems with melt fracture. Either scenario represents an economically unacceptable alternative for profile extrusion, so an improved processing polyester is needed that has both good melt strength and a high degree of shear thinning (i.e. melt fracture resistance).
Nevertheless, for some applications (e.g. flat film casting, extrusion blow molding, and foam extrusion) where the effective shear rates/stresses are already fairly low due to bigger die gaps and/or reduced extrusion rates, melt fracture may not be an issue. Thus, increasing just the melt strength alone may be acceptable. However, for profile extrusion, which typically has very high shear rates, both increased melt strength and resistance to melt fracture are important factors and should be improved simultaneously.
Chain branching is one of the most commonly used methods for improving the melt strength of a polymer, particularly polyesters. A tri-, tetra-, or higher functionality monomer is added to the polyester either during manufacture or in the processing step to create branches in the polymer. Typical branching agents for polyesters include trimellitic anhydride (TMA), pyromellitic dianhydride (PMDA), glycerol, sorbitol, hexane triol-1, 2, 6, pentaerythritol, trimethylolethane, and trimesic acid. Common applications for high melt strength polyesters include extrusion blow molding and foams.
U.S. Pat. No. 4,983,711 to Sublett describes high melt strength polyesters for extrusion blow molding applications. The polyester is poly(ethylene terephthalate) modified with 1,4-cyclohexanedimethanol (PETG) having from 0.05 to 1 mole percent of a tri-functional brancher, preferably trimellitic acid or anhydride. The 1,4-cyclohexanedimethanol (CHDM) levels are from 25 to 75 mole percent.
U.S. Pat. Nos. 5,523,382 and 5,442,036 to Beavers describe branched copolyesters suitable for extrusion blow molding. The branching agent is preferably trimellitic acid or anhydride. The copolyester contains an ethylene glycol component modified with 0.5 to 10 mole percent of CHDM and 3 to 10 mole percent of diethylene glycol. The acid component is terephthalic acid with up to 40 percent of isophthalic acid and naphthalenedicarboxylic acid.
U.S. Pat. No. 5,376,735 to Sublett describes high melt strength poly(ethylene terephthalate) (PET) for use in extrusion blow molding applications. The PET was blended with up to 3 mole percent isophthalic acid (IPA). A number of branching agents are mentioned including TMA.
Fukuda et. al. in U.S. Pat. No. 5,382,652 (reissued as RE35939) discloses a polyester resin branched with a range of tri- and tetra-functional materials including TMA and others. The resin composition is from 90 to 100 percent ethylene glycol and from 0 to 10 percent of one or more of the following: diethylene glycol, CHDM, propylene glycol, and butanediol. The application described is for improved processability around film extrusion, molding, and heat-sealability.
In U.S. Pat. No. 5,235,027, Thiele discloses a modified PET for extrusion blow molding. The PET contains from 0.5 to 5 wt. percent of isophthalic acid, 0.7 to 2.0 wt. percent of diethylene glycol, 300-2500 ppm tri- or tetra-hydroxyalkane, 80-150 ppm antimony, phosphorous of at least 25 percent by weight of the amount of antimony, red and blue toner (not exceeding 5 ppm), and various branching agents with pentaerythritol preferred. The resultant polyester has an IV between 0.8 and 1.5 dl/g.
Hauenstein in U.S. Pat. No. 4,182,841 describes a modified PET containing 12 mole percent neopentyl glycol terephthalate and 0.0062 mole percent of a polyfunctional modifying branching agent, including TMA.
Edelman et al, in U.S. Pat. Nos. 4,234,708, 4,219,527 and 4,161,579 disclose various high melt strength polyesters for extrusion blow molding. A variety of chain branching agents are utilized in amounts of from 0.025 to 1.5 mole percent with 0.25 to 10 equivalents of a chain terminator, which controls reaction conditions and prevents gelling. The importance of high zero shear viscosity coupled with shear sensitivity is also described.
Thus, there exists a need in art to have a polyester composition for use in profile extrusion which has both high melt strength to prevent sagging and excessive drawdown and a high degree of shear thinning to resist melt fracture at high processing speeds. Accordingly, it is to the provision of such that the present invention is primarily directed.
A process for producing a profile by profile extrusion comprises providing a melt of a polyester composition, which has a branching agent therein and an inherent viscosity of at least 0.65 dl/g, at a melt temperature and extruding the melt through a die to form a profile. The diacid component of the polyester composition comprises 100 to 98.0 mole percent of residues of a primary acid selected from the group consisting of terephthalic acid, naphthalenedicarboxylic acid, isophthalic acid and mixtures thereof. The glycol component of the polyester composition comprises 100 to 98.0 mole percent of residues of a primary glycol selected from the group consisting of ethylene glycol, 1,4-cyclohexanedimethanol, diethylene glycol, neopentyl glycol, and mixtures thereof. The polyester composition comprises from 0.05 to 2.0 mole percent of residues of the branching agent selected from an acidic branching agent with a tri-functional or greater monomer, an alcoholic branching agent with a tri-functional or greater monomer, and mixtures thereof. The branching agent is present as part of either the diacid component or glycol component depending on whether the functionality is acidic or alcoholic, respectively.